1. Field of the Invention
The invention relates to a method for determining the measurement uncertainty of measured values of a network analyser.
2. Discussion of the Background
Vectorial network analysers are highly-complicated measuring systems. Computation of the measurement uncertainty of the measured values is correspondingly complex. Intensive investigations have already been carried out in this field, and the topic already forms the subject matter of current research activities published, for example, in European Co-operation for Accreditation: Guidelines on the Evaluation of Vector Network Analysers (VNA), Publication Reference EA-10/12, May 2000; Rytting, Douglas Kent: “Improved RF Hardware and Calibration Methods for Network Analysers”, Hewlett Packard, Network Measurement Divisions, Santa Rosa Calif., 1991; Bachmaier, Ulrich; Hand and Stumper: “Current Problems in the Presentation of RF Measured Values”, Presentations of the 139th PTB Seminar on May 13, 1998, PTB-E58, Braunschweig, June 1998; Zeiler, Markus: “Software automates the calculation of measurement uncertainty”, metINFO, Vol. 12, No. 2/2005; Vidkjaer, Jens: “Network Analyser Uncertainty Computations for Small-Signal Model Extractions”, Technical University of Denmark, Electromagnetic Institute, Lyngby, Denmark, R549, February 1994.
One disadvantage of these known methods is that many influencing parameters, which must either be entered into the internal computer of the network analyser by the user of the network analyser in order to implement the measurement or evaluated in corresponding formulae using an external computer, contribute to the measurement uncertainty. In fact, corresponding measurement-uncertainty curves presented separately according to modulus and phase for the respectively-selected measured value, for example, the reflection factor, have already been provided for the user in the data sheet of the network analyser, as shown in FIG. 1. However, even for an experienced test engineer, it is difficult to determine and to present the associated measurement uncertainty for the relevant test result from this information. The user must select the correct measurement uncertainty curve dependent upon the measurement frequency and the respective measured value. In this context, the scales used for the display, which are typically linear scales, do not always correspond to the scaling (for example, dB-scale) required by the user for the measured result. Converting a symmetrical, linear tolerance value to an asymmetric dB-tolerance value with different upper and lower thresholds is also generally difficult. Such methods are therefore very complicated and demanding on the user. A further difficulty is that the tolerance curves for amplitude and phase are given separately, so that, in the case of a complex display, the modulus and phase values must be set off against one another in order to determine the tolerance region associated with a given measured value, as presented in FIG. 2 for a conventional, complex, graphic display.
Although vectorial network analysers are used primarily for the measurement of scattering parameters S, they can also be used for measuring Z-, Y-, H-(hybrid) parameters, group delay time, VSWR or for determining wave parameters a and b. The measurement inaccuracies (tolerances) occurring in the wave-parameter measurement are naturally larger, because there is no possibility for a preceding system-error correction by calibration in this context. With regard to wave-parameter measurements of this kind, which are preferable with nonlinear devices under test, a level calibration with an additional power meter is required and, the conventional measurement-uncertainty data previously provided by the manufacturer in the data sheet cannot therefore be used to calculate the tolerances.